Resistance welding is a common method of joining two metal pieces. This is certainly true in the assembly of electrical apparatuses such as battery packs where metal tab stock material is welded to the battery cells and to other tab pieces to create the necessary electrical interconnections. Tab stock is typically supplied on spools and cut in the factory to a specified length to make individual tabs. In general, tabs are used to electrically interconnect various parts of electrical apparatuses, and are typically steel or nickel alloys formed into a generally ribbon shaped, or an otherwise thin element.
The welding process involves contacting a pair of electrodes with the metal work pieces, in this case tabs, and while they are pressed together, passing a high level electrical current pulse between the electrodes. The current distributes through the work pieces between the two electrodes, and is most dense where the electrodes contact the work piece(s). The resistance of the metal creates heating, and because of the high power of the current pulse, the metal heats in the particular localized area between the electrodes so much that the two pieces are joined where the weld current density is highest, i.e. directly under, or between the electrodes. This is why the two pieces are pressed together. At that point, or points, what is commonly referred to as a weld nugget is produced. For the weld to be considered useful, the nugget(s) must have good mechanical strength and good electrical conductivity. There are basically two methods of resistance welding, pinch or series welding, and parallel welding.
Pinch welding involves pressing the two work pieces together from opposing side with the electrodes, and then applying the current pulse. This forces the current to travel through the interface point where the two pieces best contact each other because it provides a slightly lower resistance path. However, as the current heats up the metal at the point of compression, the path originally offering the lowest resistance heats, and increases slightly in resistance. The current starts the weld event mostly conducting through the shortest point of compression, but is continuously distributed away from the initial point as the metal heats. This causes the current density profile through the compression point to change throughout the weld event, and thus provides a single evenly welded nugget.
Parallel welding produces a pair of weld nuggets. It involves laying a first work piece over a second work piece, and applying the electrodes to the exposed side of the first piece, thereby compressing the two pieces together at two points. To produce the two weld nuggets, current must travel down from the first electrode through the first piece and the first interface point, through the second piece to the second interface point, up through the second interface point into the first piece, and into the second electrode. This method of welding requires more sophistication compared to pinch welding to produce an acceptable weld.
In pinch welding, the current must travel through the interface point to get from one electrode to the other. In parallel welding the current must have some incentive to travel through the second work piece to produce the necessary nugget integrity. This is accomplished by making the second work piece a less resistive path. There are three ways this can be achieved. First, assuming similar mechanical cross sectional areas of the two work pieces, one may make the second piece a different material than the first, such that the second piece has an inherently lower resistance. Second, assuming similar resistive characteristics of the two work pieces, increase the cross sectional area of the second work piece. Third, similar to the second, artificially decrease the resistance of the second work piece by applying a third work piece to the back of the second work piece. By adding a third work piece, the first and second solutions can be combined.
The third option is the most preferred method in the manufacture of electrical apparatuses because it facilitates rapid assembly. As such, many of the currently marketed electrical apparatuses preclude the use of pinch welding. This is due to their compact size and the added handling considerations of pinch welding, i.e., positioning two work pieces between the electrodes. To facilitate parallel welding of, for example, battery pack interconnect tabs, a metal disk, or "coin", is placed under the two tabs. After the weld operation is complete, the coin is removed and used for the next weld operation. Obviously the coin could be left under the tabs, but the incurred cost, and in the case of battery packs the increased weight, is marketably unacceptable. At the same time, although still easier to handle than a pinch weld operation, the coin method does not lend itself to automation, and limits production capability.
Therefore there is a need for a way to facilitate parallel resistance welding of thin elements without incurring cost or weight, and which accommodates automation and the rapid manufacture of electrical apparatuses.